1. Field of the Invention
The present invention relates to a distillation apparatus including a plurality of distillation columns, and for example, relates to a distillation apparatus including a plurality of distillation columns, suitable for separating a feedstock into three or more fractions by distillation.
2. Description of the Related Art
In a process for separating a feedstock containing multiple components into three or more fractions by distillation, two or more distillation columns are normally used.
For example, there is a process (referred to as a “1-butene process”) for separating 1-butene, which is useful as a raw material for linear low density polyethylene (L-LDPE), from a C4 fraction obtained by a steam cracker. In this process, components lighter than 1-butene and components heavier than 1-butene are separated from the C4 fraction to obtain product 1-butene. Note that, in the present description, “C” is used for representing a carbon number. For example, “C4” means that a carbon number is 4. To this end, there has been conventionally known a process in which, by using two conventional distillation columns, the C4 fraction is supplied to a first distillation column, an overhead fraction (containing 1-butene) of the first distillation column is supplied to a second distillation column, and product 1-butene is obtained from the column bottom of the second distillation column as described in the pamphlet “Sulzer Chemtech Distillation and Heat-Pump Technology, Production of 1-Butene from Tail Gas of a MTBE Plant” from Sulzer Chemtech Ltd.
On the other hand, a vapor re-compression system (VRC) is known as a distillation column improved in view of energy saving. In the VRC, an overhead vapor is compressed by a compressor to be increased in temperature, and the resulting fluid is used as a heat source of a reboiler of the VRC. After that, this fluid is decreased in temperature by reducing the pressure, and returned to the column top of the VRC as a reflux. It can be said that, in the VRC, the heat of the column top of one column can be fed to the column bottom of the same column by using a heat pump system. Therefore, the amount of heat supplied at the reboiler can be reduced, and energy consumption in the distillation column can be reduced.
The pamphlet from Sulzer Chemtech discloses, in a process for obtaining 1-butene from the C4 fraction by using two distillation columns, applying the VRC to each of the two distillation columns.
The pamphlet from Sulzer Chemtech also discloses a process described below, regarding the process for obtaining 1-butene from the C4 fraction by using two distillation columns. In this process, the VRC is applied to a first distillation column, and a conventional distillation column is used for a second distillation column. The C4 fraction is supplied to the first distillation column. In this column, 1-butene and a light fraction are discharged from the column top, and this fluid is pressurized and heated by a compressor. The pressurized fluid is divided into two streams, which are supplied to a reboiler of the first distillation column and a reboiler of the second distillation column, respectively, and used as heat sources of the respective reboilers. The two streams that have been used as the reboiler heat sources are joined, cooled and depressurized. The resulting fluid is then supplied to a condenser of the second distillation column, and used for cooling an overhead vapor of the second distillation column. After that, a part of the fluid is refluxed to the first distillation column, and the rest is supplied to the second distillation column as a feedstock.
U.S. Pat. No. 4,718,986 A also discloses a process for obtaining 1-butene from the C4 fraction by using two distillation columns. In this document, the VRC is applied to one of the distillation columns.
In addition to the VRC, a heat integrated distillation column (HIDiC) has been also known as the distillation column improved in view of energy saving, as described in JP H08-66601 A, JP 2004-16928 A and WO 2011/043199. In the HIDiC, by transferring heat from a rectifying section (a section above a feedstock feed position) to a stripping section (a section below the feedstock feed position) of the distillation column by heat exchange, the amount of heat supplied at a reboiler and the amount of heat removed at an overhead condenser are reduced, thereby heat efficiency is improved.